The invetnion relates to an improved electrostatic yoke for an imagining system, and more particularly to a yoke formed of two overlying conductive layers that have low electrical resistance and which adhere well to an interior surface of a glass envelope of a tube used in the imaging system.
U.S. Pat. No. 3,319,110 issued to Schlesinger on May 9, 1967 describes an electron focus projection and scanning (FPS) system which utilzes a mixed field system for focusing and deflecting an electron beam. The FPS system comprises a camera tube and an external coil. The coil provides an axially directed magnetic focus field. The tube contains an internal electrostatic yoke or deflectron formed of pairs of interleaved horizontal and vertical deflection electrodes which are attached or formed on the interior surface of the tube envelope. The electrostatic yoke generates a rotatable, bi-axial uniform electric field orthogonal to the magnetic field generated by the coil. The crossed electric and magnetic fields constitute a "focus projection and scanning" or "FPS" cavity in the central portion of the tube envelope. An FPS system provides high image resolution, high beam current density with minimum power requirements, size and weight. Conventional all-electrostatic imaging systems have an inherently long beam system and all-magnetic imaging systems are bulky, heavy and require a large amount of power. Therefore, in applications where power requirements, size and weight are to be minimized the FPS system is preferred.
U.S. Pat. No. 3,731,136 issued to Roussin on May 1, 1973 describes one configuration of an electrostatic yoke formed by depositing a thin layer of a conductive material on the interior surface of the tube envelope. The suggested methods for depositing the conductive material include spraying, evaporating and electroplating a single layer of metals.
It has been detemined that a suitable electrostatic yoke should have a resistance of about 100 ohms or less measured at the longitudinal extremes of each of the four patterns which comprise the electrostatic yoke. Additionally, the patterns must be capable of withstanding the extreme heat generated by sealing of certain of the electron gun elements into the tube envelope and must withstand abrasion from the electrical contacts of the electron gun. In order to meet these stringent requirements, the conductive layer used to form each of the patterns of the yoke was, in prior structures, frequently of sufficient thickness as to peel or flake-off the envelope. This resulted in conductive particles within the tube and, in extreme cases, in a lack of continuity in the electrostatic yoke.